Improve BaseManager, Manager types

[noelleleigh]

This PR makes a series of improvements to methods and properties of BaseManager and Manager:

• BaseManager.deconstruct(): Most of the values in the return tuple can be null (see implementation)

• BaseManager.from_queryset(): queryset_class is now generic, and the return type is made more specific (see implementation). This allows code like this to receive type information:

  class CustomQuerySet(models.QuerySet["User"]):
      ...
  
  class CustomManager(models.Manager.from_queryset(CustomQuerySet)):
      def get_queryset(self):
          qs = super().get_queryset() # ← Type checker knows get_queryset exists in the parent
          user = qs.get() # ← Inferred type is "User"
          return qs

• BaseManager._get_queryset_methods(): Return dictionary values are functions (see implementation).

• Manager._queryset_class: This property is assigned dynamically by BaseManager.from_queryset() (see implementation)

[sbdchd]

Looks good, thank you!!